The ErbB Receptor Family and its Ligands
The ErbB family of receptor tyrosine kinases couples binding of extracellular growth factor ligands to intracellular signalling pathways regulating diverse biological responses, including proliferation, differentiation, cell motility, and survival. The four closely related members of this family—ErbB1 (also known as the epidermal growth factor receptor (EGFR)/HER1), ErbB2 (neu, HER2), ErbB3 (HER3), and ErbB4 (HER4)—are activated upon ligand-induced receptor homo- and heterodimerisation. ErbB2 appears to be the preferred heterodimerisation partner for all other ErbB receptors (Tzahar et al., 1996; Graus-Porta et al., 1997).
ErbB ligands are characterised by the presence of an EGF-like domain, and can be divided into three groups on the basis of their specificity towards the ErbB receptors (Normanno et al., 2005): The first group (including EGF, TGFα and amphiregulin) binds specifically to ErbB1, the second group (including betacellulin, heparin-binding EGF, epiregulin), show dual specificity towards ErbB1 and ErbB4, whereas the third group (including the neuregulins (NRGs)) bind ErbB3 and/or ErbB4. None of the EGF-related growth factors bind ErbB2.
ErbB receptors have a broad expression pattern on epithelial, mesenchymal and neuronal cells, and signalling through these receptors plays a critical developmental role in cell fate determination in many organs (Normanno et al., 2005).
Structure and Mechanism of Activation of the ErbB Receptors
All four ErbB receptors have an extracellular ligand-binding domain, a single transmembrane domain and a cytoplasmic tyrosine kinase-containing domain. The intracellular tyrosine kinase domain of ErbB receptors is highly conserved, although the kinase domain of ErbB3 contains substitutions of critical amino acids and therefore lacks kinase activity (Guy et al., 1994). Ligand-induced dimerisation of the ErbB receptors induces activation of the kinase, receptor transphosphorylation on tyrosine residues in the C-terminal tail, followed by recruitment and activation of intracellular signalling effectors (Yarden and Sliwkowski, 2001; Jorissen et al., 2003).
The crystal structures of the extracellular domains of all four ErbBs have provided detailed insight into the process of ligand-induced receptor activation (Schlessinger, 2002). The extracellular domain of each ErbB receptor consists of four subdomains: Subdomain I and III cooperate in forming the ligand-binding site, whereas subdomain II (and perhaps also subdomain IV) participates in receptor dimerisation via direct receptor-receptor interactions. In the structures of ligand-bound ErbB1, a β hairpin (termed the dimerisation loop) in subdomain II penetrates into the dimer partner and stabilises the receptor dimer (Garrett et al., 2002; Ogiso et al., 2002). In contrast, in the structures of the inactive ErbB1, ErbB3 and ErbB4, the dimerisation loop is engaged in intramolecular interactions with subdomain IV, which prevents spontaneous receptor dimerisation in the absence of ligand (Cho and Leahy, 2002; Ferguson et al., 2003; Bouyan et al., 2005). The structure of ErbB2 is unique among the ErbBs. In the absence of a ligand, ErbB2 has a conformation that resembles the ligand-activated state of ErbB1 with a protruding dimerisation loop, poised to interact with other ErbB receptors (Cho et al., 2003; Garrett et al., 2003). This may explain the enhanced heterodimerisation capacity of ErbB2.
Although the ErbB receptor crystal structures provide a model for ErbB receptor homo- and heterodimerisation, the background for the prevalence of some ErbB homo- and heterodimers over others (Franklin et al., 2004) as well as the role of domain IV in receptor dimerisation and autoinhibition (Burgess et al., 2003; Mattoon et al., 2004) remains somewhat unclear.
The Role of ErbB Receptors in Cancer
The role of ErbB receptors in cancer is, particularly for ErbB1 and ErbB2, well documented and characterised by two main lines of evidence: Firstly, the ErbB receptors and their ligands are transforming genes in vitro and in vivo with ErbB2 showing the highest transforming potential (Di Fiore et al., 1987a, b; Shankar et al., 1989; Krane and Leder, 1996; Brandt et al., 2000; Normanno et al., 2005).
Secondly, one or more of the ErbB receptors and/or their ligands are overexpressed in the majority of solid neoplasms (for review, see Marmor et al., 2004; Normanno et al., 2005). As regards ErbB1, overexpression, gene amplification, rearrangements, or mutations of this receptor are found in multiple human malignancies, including cancers of the breast, head and neck, and lung. Accumulating evidence suggest that when ErbB1 is overexpressed, the resultant cell transformation is ligand-dependent, and several tumors show overexpression of ErbB1 together with one of its ligands, EGF or TGFα. Mutations of ErbB2 have been found only rarely, if at all, in human tumors. However, ErbB2 is frequently overexpressed in many cancers (most frequently in breast and ovarian tumors), and its overexpression is associated with poor prognosis. ErbB2 overexpression triggers spontaneous homo- and/or heterodimer formation and ligand-independent activation of the kinase domain.
Co-expression of different ErbB receptors occurs in the majority of carcinomas, and tumors that co-express different ErbB receptors are often associated with a more aggressive phenotype and a worse clinical outcome (Olayioye et al., 2000). Especially, co-expression of ErbB2 confers increased transforming potential to the other ErbB receptors, due to the fact that ErbB2-containing heterodimers show increased ligand-binding affinities, evade ligand-induced receptor downregulation and are more biologically potent (Worthylake et al., 1999; Olayioye et al., 2000). In fact, consensus is emerging that heterodimers of the ligand-less ErbB2 and the kinase-defective ErbB3 provide the most potent mitogenic and metastatic ErbB signal (Olayioye et al., 2000; Citri et al., 2003; Xue et al., 2006).
ErbB Receptor-Targeted Cancer Therapy
Due to the pivotal role of the ErbB receptors in cancer development, they are obvious targets for cancer therapy. Several anti-cancer agents targeting the ErbBs are in clinical use or development (for review, see Normanno et al., 2003; Baselga and Arteaga, 2005). They can be divided into two categories:                1. Chimeric or humanised monoclonal antibodies against the ErbB family.                    These include antibodies that prevent ligand-binding and ligand-dependent receptor activation (e.g. Cetuximab that targets the ligand-binding subdomain III of ErbB1), antibodies that interfere with ligand-independent receptor activation (e.g. Trastuzumab that targets subdomain IV of ErbB2), and antibodies that prevent receptor heterodimerisation (e.g. the anti-ErbB2 antibody Pertuzumab that targets an area around the dimerisation loop in subdomain II of ErbB2). Cetuximab is approved for the treatment of advanced-stage colorectal cancer, and is being tested in phase III trials for the treatment of squamous cell carcinomas of the head and neck and non-small cell lung cancer. Trastuzumab is approved for the treatment of metastatic breast cancers overexpressing ErbB2, and Pertuzumab in being tested in clinical phase II for the treatment of breast, ovarian, prostate and non-small cell lung cancer. However, there are limitations to the use of ErbB-targeted antibodies. For Trastuzumab, for example, the objective response rates are relatively low, and the majority of patients that benefit from Trastuzumab treatment acquire resistance within one year of treatment initiation.                        2. Small Molecule ErbB Tyrosine Kinase Inhibitors.                    The two ErbB1-specific tyrosine kinase inhibitors Gefitinib/Iressa and Erlotinib have been approved for the treatment of non-small cell lung cancer, and the dual ErbB1/ErbB2 inhibitors Lapatinib is in phase III trial for the treatment of breast cancer.                        
As an alternative to the antibody-based strategy of ErbB targeting, two recent studies have attempted to target the ErbB1 and ErbB2 by means of small peptides. The authors identified peptides, which exhibited homology to EGF-like growth factors, and which bound the ligand-binding site in ErbB1 (Nakamura et al., 2005), or an unspecified site in the extracellular domain of ErbB2 (Pero et al., 2004) thereby inhibiting ErbB1- and ErbB2-mediated mitogenesis, respectively. However, there are no reports on attempts to develop peptides that target other extracellular parts of the ErbB receptors (such as the parts involved in receptor dimerisation) and/or peptides that are capable of targeting several ErbB receptors expressed in the same tumor.